This invention is directed to microgels and in particular to microgels that are useful in cathodic electrocoating compositions.
The coating of electrically conductive substrates by an electrodeposition process (also called an electrocoating process) is a well known and important industrial process. Electrodeposition of primers to automotive substrates is widely used in the automotive industry. In this process, a conductive article, such as an autobody or an auto part, is immersed in a bath of a coating composition of an aqueous emulsion of film forming polymer and acts as an electrode in the electrodeposition process. An electric current is passed between the article and a counter-electrode in electrical contact with the aqueous emulsion, until a coating having the desired thickness is deposited on the article. In a cathodic electrocoating process, the article to be coated is the cathode and the counter-electrode is the anode.
Resin compositions used in the bath of a typical cathodic electrodeposition process also are well known in the art. These resins typically are made from polyepoxide resins which have been chain extended and then an adduct is formed to include amine groups in the resin. Amine groups typically are introduced through reaction of the resin with an amine compound. These resins are blended with a crosslinking agent and then neutralized with an acid to form a water emulsion which is usually referred to as a principal emulsion.
The principal emulsion is combined with a pigment paste, coalescent solvents, water, and other additives to form an electrocoating bath. The electrocoating bath is placed in an insulated tank containing the anode. The article to be coated is the cathode and is passed through the tank containing the electrocoating bath. The thickness of the coating that is deposited on the article being electrocoated is a function of the bath characteristics, the electrical operating characteristics, the immersion time, and the like.
The resulting coated article is removed from the bath after a set period of time and is rinsed with deionized water. The coating on the article is cured typically in an oven at sufficient temperature to produce a crosslinked finish on the article.
Cathodic electrocoating compositions, resin compositions, coating baths, and cathodic electrodeposition processes are disclosed in Jerabek et al U.S. Pat. Nos. 3,922,253 issued Nov. 25, 1975; Wismer et al 4,419,467 issued Dec. 6, 1983; Belanger 4,137,140 issued Jan. 30, 1979, Wismer et al 4,468,307 issued Aug. 25, 1984 and DebRoy et al 5,070,149 issued Dec. 3, 1991.
The use of cationic microgels in cathodic electrocoating compositions is known to solve problems of improving edge coverage and reducing edge corrosion as shown in Abbey et al U.S. Pat. Nos. 4,525,260 issued Jun. 25, 1985, Tsuchiya et al 4,788,246 issued Nov. 29, 1988, Shibata et al 4,987,178 issued Jan. 22, 1991 and Corrigan et al 5,096,556 issued Mar. 17, 1992.
However, Abbey et al and Shibata et al are both directed to acrylic polymer based microgels and Tsuchiya et al is directed to a polybutadiene based microgel. Microgels based on acrylic polymers or butadiene polymers adversely affect the general corrosion protection of coatings formed from cathodic electrocoating compositions. The microgel of this invention does not reduce the general corrosion protection of coatings formed from cathodic electrocoating compositions in comparison to the composition of the prior art. Corrigan et al forms microgels based on epoxy resins but uses a different and less efficient synthesis for forming microgels and forms microgels having a different chemical structure from those of the invention.